Housing
What size is your tank? 1 Gallon
What temperature is your tank? Usually around 70
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? Air Stone
Is your tank heated? Sometimes, but the heater isn't regulated, so I don't always have it on.
What tank mates does your betta fish live with? He's lonely

Food
What type of food do you feed your betta fish? I alternate between Betta flakes & pellets and on the weekend he gets bloodworms as a treat.
How often do you feed your betta fish? Once a day

Maintenance
How often do you perform a water change? I haven't always been the best at changing the water on a regular basis; but since I noticed a change in him, I've started changing his water 100% on a weekly basis and I try to do a 50% at least once during the week.
What percentage of the water do you change when you perform a water change? Stated above
What type of additives do you add to the water when you perform a water change? Splendid Betta Complete Water Conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters?
No, I don't know anything about how to do that or what exactly I'm testing.
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? He has a "lesion" on his one side. It looks like he's missing scales in this one spot.
How has your betta fish's behavior changed? He's become more lethargic, lying on the bottom, not eating as much.
When did you start noticing the symptoms? About 2 weeks ago.
Have you started treating your fish? If so, how? Just today, I put bettafix in, but I'm not sure if it was the best thing to do or not.
Does your fish have any history of being ill? I've only had him about 5 months.
How old is your fish (approximately)? I don't know. I rescued him from a wedding center piece On June 16th of this year.
I'm really worried about my little guy. I love him dearly. I know I haven't been the best fishy mommy, but I'm really trying. I've read a lot of the posts on here and have just become more confused and frustrated. I went to PetSmart today to get aquarium salt and the really nice girl there who said she has a betta and knows a lot about them, recommended Bettafix. I looked at forums on here when I got home to make sure I administered it properly so as not to injure him and it seems everyone is saying not to use it. But if it's sooo bad, why would they make it. I asked a friend who used to breed them, and she said he looked old. I'm soo frustrated and afraid of losing my little guy. I'm desperate for help!!!
I've attached a picture of what he looks like tonight.

Bettafix contains an ingredient harmful to bettas. I'd use aquarium salt to treat physical injuries. They make it because it may or may not help them recover from certain things, but it will harm them in other ways. It's sort of like how they make food that has a lot of fillers and almost no meat, even though bettas are carnivores or how they make tiny .5 gallon tanks with dividers or have tanks that are tall and skinny and don't have a lot of surface area that say "perfect for bettas" on the box... It's cheap. People buy it. They often don't care if they're making money off of it.
The spot looks grey to me, but it may just be the picture. When you say you noticed symptoms about two weeks ago, did you mean the lesion or the lethargy and lack of interest in food?

When I first started noticing symptons, it was just the lethargy at first. I got up in the morning and said good morning to him and he didn't respond. I didn't think he was going to make it through the day, but he's still around. His appearance looked fine and he would still get excited at dinner time. As time has progressed, his appearance changed. The spot on his side has become more pronounced. Also, he doesn't really seem to be eating.
I had given him a little less than the recommended dose on the bettafix bottle last night and he actually seemed to respond a bit better when I said hi after work today. A friend who used to breed bettas recommended taking a lot of his water out so he could get to the surface a little easier. I returned the bettafix today and got him Aquarium salt. I'm just not sure exactly how to administer it. He's a tough little guy and I want to give him every chance I can to get back to his old self. I don't want to do anything that could make him worse. When I use the salt, should I change his water completely first (He's still in the bettafix water from last night) Is it okay to put in his tank while he's still in there?

He's freezing and 1 gallon is too small to keep a betta healthy for very long, unfortunately.

You need to get him in a larger tank you can keep properly warm. Until then he's unlikely to recover.. 70 degrees is way too cold for bettas. He needs to be kept a minimum of 76, but ideally more around 78/79 area. You'll need to acclimate him to the new water very slowly so you won't shock him.

It looks like he may have the beginnings of a fungus or Columnaris but it's not fuzzy is it? Just gray? Could he have rubbed him on something in the aquarium?

I would probably just treat with aquarium salt and clean water water at this point, but watch him that it doesn't appear to be getting worse.

Right now a 1 gallon is what he may just need... However the temperature preferably needs to be 78 minimum, but at least try to get it to 76 especially if it ends up fungal (fungal infections like warm water)

1 gallon bowls and tanks should be cleaned once a day, perhaps once every two days. Right now since he is sick look to doing a full water change every single day. Try to get a temporary heating solution: room heater, bowl heater, hot water bottles all around his tank, towels wrapped around his tank, put his tank higher and away from windows, doors and hallways.

Aquarium salt pre-dissolved (1-3 teaspoons per gallon) added to the tank, and whatever you take out during water changes re-added every day for no more than 7-14 days. This will help boost the immune system plus does help fight off infections.

What ornaments are in the tank? It could just be scales rubbed off, otherwise could be a start of a fungal bacteria which does not always start off with fuzz... but sometimes just with lesions.

Thank you everyone for your help! I'm just about to clean out his tank and add the salt. Right now, 1 gallon is all he gets. If he makes it through Christmas, I have a bigger tank, I just don't have a proper heater for it and its just not something I can spend money on right now. I plugged in his heater when I got home tonight and I turned on my fireplace. (Don't worry, he's nowhere near it.) But I have a small basement apartment and it heats the whole place up nice and toasty. With the fireplace on and his heater, his water temp is currently at a balmy 80. He has a small cheap heater that doesn't regulate his temperature other than me plugging it in and unplugging it. Oh, and his tank has gravel, a rounded edge thermometer, 3 fake plants, (all of which I tested for sharp edges), a pump and a heater. There are no sharp edges in his tank, but his "spot does kinda look like he rubbed scales off.

So, I'm wondering when I change his water and put the salt in, should I still put water conditioner in or just the salt. Also, should I turn his pump back on. I've had it off for a day or two.

In a smaller space, he could have just darted into a plant and scraped off some scales. Even with the best of plants, that can happen :) Always add conditioner no matter what.

As for the pump, if it is regulated and does not shove him around you can turn it on or leave it up - it's your preference.

When you are not home (heater is off) try putting a towel around the tank, and plastic wrap on the top of the bowl (with a hole so he can breath of course) which will keep it humid, and warmer, much longer.